There are many documented cases of poorly specified control valves. Some of these valves do not operate adequately because of excessive head loss or cavitation during steady-state flow conditions; others are inadequate to control hydraulic transients because of poor valve selection or poor operation. When specifying valves for flow control and/or pumping stations, the engineer must carefully evaluate the type, number, and size of valves to provide adequate steady and transient flow regulation.
Note: Even with a comprehensive understanding of the system equipment and operations, the engineer should realize that it may not be possible to precisely model the actual system and system components. Therefore, it is the engineer's responsibility to recognize these modeling limitations, use appropriate safety factors, and apply good engineering judgement when performing transient analysis.
The advantage of surge-relief valves is that they are relatively inexpensive and easy to fit into a pumping system at the locations of interest. Generally, valves control surge conditions by opening and/or closing according to preset characteristics. This restricts hydraulic transients to more tolerable limits, but it can rarely eliminate cavitation or water-column separation. Moreover, if the valves are oversized or operated too rapidly, other types of water hammer problems may result (e.g., water bleeding, and excessive flow reversals), possibly resulting in worse transients than without valve protection. However, with careful Bentley HAMMER V8i Edition modeling and design, valves offer a versatile and powerful means to safely control water hammer.
The following are different types of surge-relief valves:
The following descriptions and figures show their geometry and schematics:
Check valvea check valve is commonly installed in a municipal pumping station to prevent flow from reversing through the pump. A dashpot may be provided to avoid check valve slam; however, surges still may occur in the piping system and other methods may also be required. A check valve equipped with an electronically controlled closure device is often used by engineers. The timing and rate of closure must be carefully set to protect both the pump and the discharge system.
Pressure-relief valveThis valve is usually installed across the pumps and discharge headers or at critical points along the pipeline. It opens when a preset pressure is exceeded and closes immediately after pressure drops below this setting. A damped closure may be provided to allow for a longer closing time. One of the main concerns is the considerable time lag for the valve to open following a power failure. Transient pressure waves can come and go in a fraction of second. Very often, this valve is used as a redundant measure, to limit the pressure rise during normal pumping operations.
Pump station bypass with check valveIf the suction water level is high, a bypass line can slow the reduction in flow by supplying water to the pipeline during the downsurge period (following a power failure) using potential energy in the suction reservoir. However, it provides no upsurge protection to a pumping system because no back flow is allowed through the check valve. It can be effective in a downhill or flat pipeline.
A smaller bypass line is sometimes provided (as shown by dotted lines) around the check valve in the primary bypass line.
Inline bypass with check valveThe check valve is usually located downstream of the location of cavitation at a high point. The bypass line should be sized so that no high pressure is built up at the downstream section and no large reverse-flow velocity occurs in the upstream section of the check valve. Normally, an air valve needs to be installed at the crest to eliminate vapor pressure, and a surge-anticipator valve is located at the pump station to protect it and the pipe section between the pump and the high point.
Air-inlet (vacuum-breaker) valveThis valve consists of an orifice that can be opened or blocked based on system pressure, often by a float device. When pressure drops below the valve elevation, air is sucked in quickly through the inlet orifice to maintain atmospheric pressure. If the opening is too small, the incoming air velocity may reach the sonic limit, resulting in subatmospheric pressure inside the system. This valve does not allow air to escape the system; it must exit farther down the line.
Air-release valveThis valve also consists of an orifice equipped with a mechanism to open or close it, often by a float device. When air accumulates inside the valve body, or reaches a preset residual volume, air is released from the valve in an orderly and gradual manner. Air is not allowed to enter the system. This valve is commonly installed at all local high points within the water system.
Combination air valveCombination air valves consist of at least two components: a) a large air inlet valve, b) a large outlet orifice (two-way), and possibly a restrictor of some kind to reduce the opening to a much smaller orifice (three-way) when air in the valve body is less than the residual volume. When pressure drops below the elevation of the valve, air enters quickly through the vacuum breaker to maintain the pressure near atmospheric. Upon the upsurge, air can be expelled quickly through the bigger outlet, until the air in the system is almost totally removed and water starts to enter the valve body. The remaining air volume inside the valve is released in a controlled manner by the small outlet orifice, acting as an air cushion to reduce the transient pressure rise.
This type of valve is popular both for water-distribution systems and sanitary forcemains. However, if the air volume allowed into the pipe system is big and, if it is released too quickly, excessively high transient pressures can occur when the two water columns accelerate towards each other during a prolonged period of air release. The static head can defeat the effectiveness of the air cushion due to the large buildup of momentum in these accelerating water columns.
Hydraulically controlled slow-closing air valvesThis valve is located at high points of the piping system and acts like an air-inlet valve and surge-anticipator. When line pressure at the valve drops below atmospheric pressure, it admits air into the pipeline. Upon upsurge, air, water, or a mixture of air and water can bleed out to the atmosphere. One of the drawbacks of this installation is the need for a piping system to drain water away.
Surge-anticipator valveThe surge anticipator is normally installed across the pump suction and discharge headers, with suitable connecting piping. It opens quickly at a specified time after power failure (or at a preset low-pressure limit) to allow flow to begin before the main upsurge returns to the pump station, then closes slowly at a preadjusted rate. During the valve-closing period, flow may decrease much more rapidly than the opening area of the valve. High flow velocities in the pipeline can prevent a hydraulically actuated SAV from closing, in extreme cases. Consult the valve manufacturer's catalog to select the correct valve type, size, and piloting (if applicable) for your application.
Rupture diskA rupture disk is equipped with a membrane which can burst to discharge a large flow rate and relieve mass (pressure) from the system whenever transient pressures exceed a pre-set value. Such disks may rupture at a different pressure and both the upper and lower burst limit provided by the manufacturer should be modeled using Bentley HAMMER V8i Edition.
Pressure-sustaining valveThis valve is usually installed at the downstream end of a pump-discharge line. It dissipates large amounts of energy just before flow drains to a lower-energy water system. The valve sustains a stable pressure to the upstream, higher-head system, by adjusting the opening area of the valve multi-orifices. However, during the transient period, this valve cannot physically tune the orifices fast enough to catch rapid pressure changes.
A sample run based on a case study is presented in the following figure. As shown, the combination air valve does not help to control surge due to the big air pocket and the high head at the downstream reservoir, in this particular case.
Figure 14-20: Bentley HAMMER V8i Edition Results for a Combined Air Valve
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